The usage of light-emitting diodes (LEDs) to provide illumination is increasing rapidly as the cost of LEDs decrease and the endurance of the LEDs increases to cause the overall effective cost of operating LED lighting products to be lower than incandescent lamps and fluorescent lamps providing equivalent illumination. Also, LEDs can be dimmed by controlling the current through the LEDs because LEDs are current driven devices. The current through a plurality of LEDs in a lighting device must be controlled tightly in order to control the illumination provided by the LEDs. Typically, the secondary of an LED lighting device must be electrically isolated from the primary (line and neutral side) of the lighting device to meet applicable safety standards (e.g., IEC class II isolation). In addition, an LED driver circuit should have a high power factor and should have a constant current control.
One known solution to the foregoing requirements is to use a flyback converter, as shown in FIG. 1, to produce the DC in the secondary from the primary source. The flyback converter in many known configurations provides power factor correction to produce a high power factor, and provides isolation between the primary and secondary circuits. By using primary current sensing techniques to control the secondary current through the LEDs, the flyback converter provides an LED driver that is low in cost when compared with other topologies.
However, there is at least one known drawback for the conventional flyback converter. When the load and dimming range is wide, the flyback converter will typically work between a continuous mode and a pulsing mode (shown in FIG. 2 and FIG. 3). The main reason for this phenomenon is that when the output power is too small, the duty ratio becomes very small and the operating frequency becomes very large.
Unfortunately, in LED driver applications the pulsing mode is highly undesirable because an LED load is very sensitive to variations in the voltage, and as a result the LEDs might flicker.
An exemplary isolated LED driver 100 includes a primary side circuit 110 and a secondary side circuit 120. The exemplary LED driver 100 as further discussed herein may be referred to as a flyback converter 100. In the flyback converter 100 of FIG. 1, a load R_load (i.e., array of one or more LEDs, arranged in series and/or in parallel) is located on the secondary side circuit 120 with secondary ground GND_S. The primary side circuit 110 has its own ground, primary ground GND_P. An input voltage V_in could be provided either from an input rectifier such as a diode bridge (not shown) or from a power factor correction circuit (not shown) output. Accordingly, the input voltage V_in is generally characterized herein as a DC voltage supply.
As shown in FIG. 1, the LED driver may include a transformer T1. The transformer T1 includes a primary winding T1_P and a secondary winding T1_S. Transformer T1 may be a flyback transformer as is conventionally known in the art to help provide class II isolation and to provide power conversion. The primary winding T1_P includes a first primary side terminal T1_P1 and a second primary side terminal T1_P2. The secondary winding T1_S includes a first secondary side terminal T1_S1 and a second secondary side terminal T1_S2.
The primary side circuit 110 of the LED driver 100 includes one or more of: the input voltage V_in, the primary winding T1_P, a first capacitor C1, a constant current control integrated circuit (IC) 112, a first switching element Q1, and a first resistor R1.
The input voltage V_in may be coupled with the first primary side terminal T1_P1 of the primary winding T1_P. The first capacitor C1 may be coupled between the input voltage V_in and the primary ground GND_P. The first capacitor C1 may be used as a filtering capacitor C1. The first switching element Q1 may be referred to as a first switch Q1. The first switch Q1 includes a drain node Q1_d, a gate node Q1_g, and a source node Q1_s. The constant current control integrated circuit (IC) 112 (e.g., an SY5801 device as provided by Silergy Company) may include a DVR (gate driver signal output) pin, a PWM (dimming control input) pin, and an ISEN (primary current sensing) pin. The constant current control IC 112 may be used to control the first switching element Q1 by coupling the DVR pin to the gate node Q1_g of the first switch Q1. The PWM pin of the constant current control IC 112 may be coupled to a dimming control input Dim_ctl. The dimming control input Dim_ctl may for example be provided from a wired or wireless external dimming device and typically gives a reference output current level, wherein the constant current control IC 112 adjusts the operating frequency and duty ratio according to an internal algorithm to maintain a constant output current on the secondary side circuit 120.
The second primary side terminal T1_P2 of the primary winding T1_P may be coupled to the drain node Q1_d of the first switch Q1. The first resistor R1 may be coupled between the source node Q1_s and the primary ground GND_P. The first resistor R1 may be a current sensing resistor R1. The ISEN pin of the constant current control IC 112 may be coupled to at least one side of the first resistor R1, and the constant current control IC 112 is further coupled to the primary ground. The constant current control IC 112 uses a primary sensing technique to sense the primary current going through the switching element Q1 and control the current going through the load R_load of the secondary side circuit 120. The constant current control IC 112 controls the switch Q1 to force the input current received at the secondary side 120 to follow the input voltage V1 waveform to achieve a high power factor while maintaining target output current and voltage settings in response to the dimming control input Dim_ctl.
The secondary side circuit 120 of the illustrated exemplary LED driver 100 includes a first diode D1, a second capacitor C2, and the load R_load. The second capacitor C2 may be referred to as an output buffer capacitor C2. The first secondary side terminal T1_S1 of the secondary winding T1_S may be coupled to the secondary ground GND_S. The second secondary side terminal T1_S2 of the secondary winding T1_S may be coupled to the anode of the first diode D1. The second capacitor C2 may be coupled between the cathode of the first diode D1 and the secondary ground GND_S. The first diode D1 may be a rectifier diode D1 configured to allow the energy from the secondary winding T1_S of the transformer T1 to charge the first second capacitor C2 when the first switch Q1 is off. The load R_load may be coupled in parallel with the second capacitor C2 between the cathode of the first diode D1 and the secondary ground GND_S. An output voltage V_out may be measured across the load R_load.
As shown in FIG. 2, when the load R_load is moderate to heavy, the transformer T1 of the LED driver 100 will work in a continuous mode. In the continuous mode, the signal from the DRV pin is continuous. As a result, the second C2 will get charged up in a regular pattern which keeps ripple voltage small enough to not cause any big ripple current through the load.
As shown in FIG. 3, when the load R_load is light (i.e., the load is small, due for example to a highly dimmed LED output) the transformer T1 of the LED driver 100 will work in a pulsing mode. The transformer T1 pulses due to a high operating frequency combined with a small duty ratio. As shown in FIG. 3, the signal from the DRV pin of the constant current control IC 112 starts to pulse as a result of the second capacitor C1 getting charged up periodically, which causes a relatively large ripple voltage on the second capacitor C2. The large ripple voltage causes even larger current ripple through the load R_load, which leads to flickering of the load R_load. One of skill in the art may appreciate that flickering is typically not acceptable for LED lighting applications.